This invention relates to a method of making a cast single crystal, to the mould used in this method and to a cast object made using this method or mould.
The manufacture of cast single crystals has been practised for many years, initially as a source of laboratory specimens and more recently to provide objects such as single crystal turbine blades which take advantage of the improved and anistropic properties available with this form of metal. The method of manufacture has varied over the years; initially a seeding technique was used for laboratory purposes but while suitable for the labour-intensive, small scale manufacture needed for this environment, the technique was not thought to be suitable for large scale construction or semi-automated manufacture.
The first alternative to be considered involved the use of a starter chamber in which molten metal is caused to solidify in a columnar grained form, and a restrictor of small opening which is intended to allow only one grain to grow into the mould cavity proper. This was found not to select one grain effectively, and an inclined passage selector between the starter and the main cavity was proposed. However, since face-centred cubic crystal materials will grow in their &lt;100&gt; directions preferentially, a passage way lying in a single plane will still be able to allow two or more crystals to grow through if the &lt;100&gt; directions lie in the plane of the passage way.
The final development of this selector concept was therefore to use a helical passage which has been found to be successful in blocking the passage of all but one grain into the main mould cavity. However, this technique has the disadvantage that the orientation of the crystal produced is only determined in one direction. This is the direction of growth of the crystal, normally determined by the heat flow from the mould. This direction will correspond, in the most common face-centred cubic case, to the &lt;0,0,1&gt; direction. At right angles to this direction the orientation of the crystal will be random.
This randomness can have undesirable consequences. Thus the vibrational characteristics of a component will be dependent on the physical properties such as the elastic and shear moduli. Since a face-centred cubic single crystal is highly anisotropic it is predictable that the vibration characteristics of a component, e.g. a turbine blade for a gas turbine engine, will depend upon the crystal orientation about the blade stacking axis. For some objects such as blades, the random variation in vibrational characteristics due to orientation about the axis may be embarrassing and may prevent exploitation of the properties of the single crystal to the full.
If a speed crystal is used instead of a selector to produce the single crystal casting, the longitudinal and transverse orientations of the single crystal may be predetermined by arranging the orientation of the seed crystal. In order to ensure epitaxial growth from a seed it is essential that nucleation does not occur from other features such as mould walls. To ensure that this does not happen the mould must be heated above the melting point of the metal, and clearly the seed crystal must not melt completely. In the past it has been difficult to reconcile these conflicting requirements in a manner which satisfies the needs of a production process. We have invented a method, and a mould for use in this method, which enables the use of a seed crystal while reducing the danger of extraneous nucleation.